1. Field of the Invention
The present invention relates to pressure pulse generators in general, and in particular to pressure pulse generators such as the "mud siren" type used in oil industry MWD (measurements-while-drilling) operations to transmit downhole measurement information to the well surface during drilling by way of a mud column located in a drill string.
2. Description of the Prior Art
Many systems exist for transmitting data representative of one or more measured downhole conditions to the surface during the drilling of a well borehole. One such system, described in Godbey U.S. Pat. No. 3,309,656, employs a downhole pressure pulse generator or modulator and is operated to transmit modulated signals carrying encoded data at acoustic frequencies to the surface by way of the mud column in the drill string. In such a system, it has been found useful to power the downhole electrical components by means of a self-contained mud-driven turbine generator unit (known as a "mud turbine") positioned downstream of the modulator.
Existing modulators of the mud siren type usually take the form of "turbine-like" signal generating valves positioned in the drill string near the drill bit and exposed to the circulating mud path. A typical such modulator is comprised of a fixed stator and a motor-driven rotatable rotor, positioned coaxially of each other. The stator and rotor are each formed with a plurality of block-like radial extensions or lobes spaced circumferentially about a central hub so that the gaps between adjacent lobes present a plurality of openings or ports to the oncoming mud flow stream. When the respective ports of the stator and rotor are in direct alignment, they provide the greatest passageway for flow of drilling mud through the modulator. When the rotor rotates relative to the stator, alignment between the respective ports is shifted, interrupting the flow of mud to generate pressure pulses in the nature of acoustic signals. Rotation of the rotor relative to the stator in the circulating mud flow produces a cyclic acoustic signal that travels up the mud column in the drill string to be detected at the drillsite surface. By selectively varying the rotation of the rotor to produce changes in the signal, modulation in the form of an encoded pressure pulse is achieved which carries information from downhole instruments to the surface for analysis.
The lobe configuration and the relative placement of the stator and rotor elements of conventional modulators is such as to subject the rotor to fluid dynamic forces due to the mud stream that cause the rotor to seek a "stable closed" position in which the lobes of the rotor block the ports of the stator. There is thus an undesirable tendency for the modulator to assume a position that blocks the free flow of drilling mud whenever the rotor becomes even temporarily inoperative. This increases the likelihood that the modulator will jam, as solids carried by the mud stream are forced to pass through restricted modulator passages. Rotor restart is made more difficult because the reduced mud flow interferes with the generation of rotor power by the mud turbine below. Prolonged modulator closing can obstruct mud flow to such an extent that lubrication of the drill bit and other vital functions of the mud become so adversely affected, that the entire drilling operation is jeopardized.
A number of approaches have been proposed to solve the problem caused by the tendency of existing modulators to assume the closed position described above. One such approach, described in Patton, et al. U.S. Pat. No. 3,792,429, is to use magnetic force to bias the modulator toward an open position and hold it there in the event the rotor becomes inoperative. Magnetic attraction between a magnet attached to the modulator housing and a cooperating magnetic element positioned on the rotor shaft develops sufficient torque to overcome the fluid dynamic torque caused by the drilling mud stream. This approach has the disadvantages that the tool must be lengthened to accommodate the magnets and that introduction of an extraneous magnetic field downhole can interfere with measurements of the earth's magnetic field (used to derive tool orientation).
In commercial MWD operations, the spacing between the rotor and stator components of the modulator must be narrow in order to produce satisfactory acoustic signals. This requirement makes the modulator particularly susceptible to jamming or obstruction by solids present in the mud stream. A system for avoiding such jamming, described in Manning U.S. Pat. No. Re. 29,734, includes control means responsive to conditions tending to slow the motor (such as an increase in pressure differential across the modulator or an increase in driving torque requirement) for temporarily separating the rotor and stator in order to allow debris to be cleared from the modulator by the flowing mud. Such a system can be employed to provide some relief from the decreased mud flow experienced with a closed modulator by separating the modulator parts in response to the pressure differential increase experienced when the modulator assumes a closed position.